Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Formation conditions of ultrabasic and basic dike rocks from Yllymakh and Ryabinovy Massifs, Central Aldan

Rokosova E. Yu.

Novosibirsk State University, Novosibirsk, Russia

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

rokosovae@gmail.com

Yllymakh and Ryabinovy alkaline massifs (Central Aldan) of Mesozoic age belong to an intricate volcano-plutonic structures. These massifs consist of intrusive, volcanic and dike rocks of potassic series. They are about 40-50 km² in area. In these massifs, almost the entire range of rocks was found, from the potassic alkaline ultrabasic rocks through basic and intermediate ones to alkaline granosyenites and granites. The formation of massifs related to the activity of the Mesozoic rift structure of the Aldanian shield. The location of the ultrabasic and basic rocks of Ryabinovy massif is limited, mainly, within the alkali-picrite neck at the north-eastern side of the massif. The ultrabasic rocks of the Yllymakh massif are not outcropped and are located at a depth. This is confirmed by the presence of nodules and xenoliths of the pyroxenite and wehrlite in basic rocks (Kostyuk et al, 1990).

We have studied dikes of the shonkinite-picrites which are located within the alkali-picrite neck of the Ryabinovy massif and alkaline basic dike rock of the Yllymakh massif. Shonkinite-picrites consist of (vol.%) 56-51 clinopyroxene, 25-30 Fe-Mg mica, 10-12 potassium feldspar, 4-3 apatite, 2 magnetite, 2-3 sphene and rutile. Alkaline basic dike rock consist of (vol.%) 60 clinopyroxene, 15 olivine, 10 pseudoleucite, 2 K-Na feldspar, 3 magnetite and sulfides, 10 glass. The alkaline basic dike rock differ in bulk chemical composition from the shonkinite-picrites in higher contents of (wt.%) SiO₂ (50,2 vs 43,4), Al₂O₃ (12,5 vs 5,5), Na₂O (2,7 vs 1,6), K₂O (5,1 vs 3,6) and lower Fe₂O₃ (10,4 vs 15,5), MgO (7,7 vs 10,6), CaO (9,3 vs 14,2).

Clinopyroxenes of the considered rocks are evenly colored (yellowish-green, dark-green) and zoned. In zoned clinopyroxenes, cores are yellowish-green and rims - dark-green. The content of the FeO and Na₂O increases from cores to rims in the zoned clinopyroxenes. The clinopyroxenes of the alkaline basic dike rock differ in composition from the clinopyroxenes of the shonkinite-picrite in higher contents of (wt%) Al₂O₃ (2-4,7 vs 0,65-1,3), MgO (12,9-15,8 vs 9,1-13,6) and lower Na₂O (0,6-1,1 vs 1,8-5,3), FeO ( 5,5-9,6 vs 7,9-14,7). The clinopyroxenes of the alkaline basic dike rock, according to international classification, are ascribed to augite and the clinopyroxenes of the shonkinite-picrite are ascribed to diopside and aegirine. Clinopyroxenes of the considered rocks, except for Ba and Nb, are significantly enriched in trace and rare elements relative to primitive mantle. It should be noted that the rims of zoned phenocrysts have higher contents of trace elements than their cores. Clinopyroxenes are more enriched in LREE and less enriched in HREE. It is generally accepted that clinopyroxene accumulates LREE during melting of garnet-spinel facies mantle.

By studying the primary melt inclusions in the clinopyroxenes of the shonkinite-picrite of the RYABINOVY massif, it was found that at the crystallization stage of clinopyroxenes the melt was heterogeneous and consisted of immiscible silicate, silicate-carbonate, carbonate-salt, and carbonate fractions. The carbonate-salt and carbonate melts separated from silicate magma were enriched in Ca, alkalies, CO₂, S, Cl and were an immiscible fraction of carbonatite melt. The temperature of the heterogenization of melts is above 1120-1170 °C. The composition of the silicate melts in the homogenized inclusions in diopside of the shonkinite-picrite corresponds to tephrite. The contents of Si, Al, alkalis increases, and the contents of Mg, Fe, Ca decreases in the melts of the homogenized inclusions from the later mineral – aegirine, i.e. the composition of melts has evolved towards of the alkali melaphonolite-trachytes.

By studying the glassy and partly crystallized melt inclusions in minerals of the alkaline basic dike rock of the YLLYMAKH massif, it was found that compositions of the unheated silicate glasses from primary inclusions in clinopyroxenes and secondary inclusions in olivine are enough highly siliceous (56.4 - 62 wt.% SiO₂) and are close to the composition of the alkali pulaskite developed on the massif (Kostyuk et al, 1990). The average composition of unheated primary glassy inclusions in Ti-magnetite contains up to 53 wt. % SiO₂, 23.6 wt. % Al₂O₃, 10 wt. % alkalis and is close to the composition of nephelinic syenite (tinguaite), which are also developed on the massif. The unheated glass of the primary inclusions in apatite have highly siliceous, monzonitic composition. The composition of secondary heated inclusions in olivine showed only insignificant changes with respect to unheated inclusions, in particular, insignificant decrease in Si and alkali contents, more significant decrease in Al, and significant increase in Mg, Fe, and Ca. The composition of heated inclusions in olivine is close to the composition of the pseudoleucite phonolite. The composition of primary and secondary homogenized inclusions in clinopyroxenes as compared to unheated inclusions are significantly enriched in Mg and Ca at very insignificant decrease of Si, alkalis, Fe. The composition of primary homogenized inclusions is close to the composition of basic feldspathoid rocks – malignites, and the composition of secondary homogenized inclusions – pseudoleucite phonolite-monzonite. The secondary melts in olivine and primary melts in clinopyroxene contained extremely low water content of 0,01-1,13 wt.% and were enriched in BaO (0,8-0,1 wt.%), Cl (0,1-0,3 wt.%) and trace elements (especially LREE and MREE), whose abundances are several times higher than mantle values.

Fig.1 The evolution trend of the melts involved in the formation of rocks of the Yllymakh and Ryabinvy massifs.

Yllymakh massif. Rocks: 1 - pulaskite, 2 - pseudoleucite phonolite, 3 - malignite, 4 - studied alkaline basic dike rock with subtracted olivine; 5 - studied alkaline basic dike rock. Melt inclusions in the minerals from alkaline basic dike: 6 - unheated inclusions in olivine, 7 - heated inclusions in olivine, 8- unheated inclusions in pyroxene, 9 - heated inclusions in pyroxene, 10- unheated inclusions in Ti-magnetite, 11 - unheated inclusions in apatite.

Ryabinovy massif. Rocks: 12 - minette, 13 – shonkinite-picrite and shonkinite. Melt inclusions from shonkinite-picrite: 14 - heated and unheated inclusions in pyroxenes.

The composition of heated and unheated inclusions in minerals of the studied rocks of the Yllymakh and Ryabinovy massifs define a common evolution trend of the melts (fig. 1). It is seen that crystallization of the melts was accompanied by the decrease in the contents of calcium, iron, magnesium, and increase in aluminum, silica, alkalis, which is typical of the evolution of alkaline-basaltoid melts. The studied alkaline basic dike rock, malignites, monzonites, pseudoleucite phonolites, pulaskites of the Yllymakh massif and shonkinite-picrites, shonkinites, minettes of the Ryabinovy massif are also plotted in the trend. Such a trend may indicate that studied rocks of the Yllymakh and Ryabinovy massifs crystallized from parental magmas with similar alkali-basalt composition. Undoubtedly, the entire rock series developed at the Yllymakh massif and dikes of shonkinite-picrites, shonkinites, minettes of the Ryabinovy massif was derived via fractional crystallization of parental alkaline basaltoid magmas. The shonkinite-picrites of the Ryabinovy massif crystallized at an earlier high-temperature stage of evolution of the parental magma, and alkaline basic dike rock of the Yllymakh massif - at a later stage.

It should be noted that during the crystallization of the studied rocks, probably, the crustal material/crustal fluids was involved in the magmatic process. This is evidenced by the fact that spider-diagramm of the shonkinite-picrites, shonkinites, minettes of the Ryabinovy massif shows negative Ta, Nb anomalies, and these rocks have high La/Nb ratio (La/Nb: 14,1 – for shonkinite-picrites, 5,5 – for shonkinites and 8,9 – for minettes). In the Nb/Ta–La/Nb diagram with shown fields of mantle and crustal sources (Barth et al., 2000), glassy melts conserved in the olivine and clinopyroxene rims are plotted in the influence zone of continental crust (Panina et al., 2011).

References:

Kostyuk V. P., Panina L. I., Zhidkov A. Ya., et al.. Potassic Alkaline Magmatism of the Baikal-Stanovoy Riftogenic System . Novosibirsk. Nauka. 1990. 235 P.

Panina L.I. , Nikolaeva A.T., Rokosova E.Yu. Crystallization conditions of the alkaline-basic dike from the Yllymakh Massif, Central Aldan: Evidence from melt inclusion data in minerals. Geochemistry. 2011. V. 49 (2). P. 120-138

Barth M.G., McDonough W.F., Rudnik R.L. Tracing the budget of Nb and Ta in the continental crust. Chem. Geology. 2000. V. 165. P. 197–213.